This invention is directed to the preparation of pigmentary quinacridone in the .alpha.-, .beta.-, .gamma.-form.
Quinacridone is a well-known compound and is commonly prepared by three typical methods, i.e., (1) cyclization of 2,5-diarylamino-terephthalic acids, (2) oxidation of dihydroquinacridone, and (3) reduction of quinacridonequinones, described in Chem. Rev., Vol. 67No. 1Labana et al., Quinacridones, pages 3- 5. The quinacridone which results from these processes, referred to as crude quinacridone, is generally unsuitable for direct use as a pigment and must be subjected to further processing to reduce the particle size and produce a definite crystal form of quinacridone.
The .alpha.-, .beta.-, and .gamma.-forms of pigmentary quinacridone are well-known and widely used commercially. The .alpha.-form is prepared by precipitation of solution of crude quinacridone in sulfuric acid, known as acid pasting, or upon its being heated with aqueous potassium hydroxide, as described, for example, in Japanese Patent 20,073, Chem. Abstr., 62, 10437 (1965). Crude quinacridone or other forms of quinacridone is converted to the .alpha.-form by dry ball-milling with sodium chloride, e.g., Reidinger et al. U.S. Pat. No. 2,844,484. The .beta.-form is prepared by ball-milling crude quinacridone or other forms of quinacridone with sodium chloride in the presence of an aromatic hydrocarbon, e.g., Struve U.S. Pat. No. 2,844,485. The .gamma.-form is prepared by ball-milling crude quinacridone or other forms of quinacridone with sodium chloride in the presence of dimethylformamide, e.g., Manger et al. U.S. Pat. No. 2,844,581.
All of the commonly used methods for converting crude quinacridone to a pigmentary and definite crystal form, described above, employ large quantities of strong acid or inorganic salt and milling which necessitates further processing steps to separate the pigment from the acid, salt and mill.
This invention provides for a process for preparing pigmentary quinacridone in the .alpha.-, .beta.-, or .gamma.-form which eliminates the need for acid pasting or milling.